The present invention relates to a single protection device, including a transformer, an input circuit, and a switch circuit, suitable for addition to an electrical power system that includes a control panel electrically interconnected with a starter to control an electrical device. It is desirable to monitor the performance of devices such as electric motors and to control them through remotely located control panels. While this has long been possible, it has been costly to install the required sensors and controls.
Springer, U.S. Pat. No. 4,885,655 discloses an integrated capacitor-start, induction-run electric motor starter and protection circuit specifically suitable for a water pump. The circuit requires physical electrical connection to the power cables to sense the phase angle between the voltage and the current applied to the motor to indicate when it is operating without a load. When such a condition occurs, the circuit triggers an activator coil to de-energize the motor by opening a switch member. The integrated starter and protection circuit is suitable only for induction machines.
Libert, U.S. Pat. No. 4,887,018, discloses a start-up circuit for gradually starting a multiphase motor and which also includes detector circuits for detecting various fault conditions and for disconnecting power from the motor in response to fault conditions. Integrating the motor starter and protection circuit into one device makes it unsuitable for addition to existing power systems to detect fault conditions within power cables.
Flückiger, U.S. Pat. No. 5,359,273, discloses a load control circuit for controlling the load of an asynchronous motor. The circuit includes a pair of capacitors, one of which can be connected in parallel with the other by a switch. Control over whether one or two capacitors is supplied as a load to the motor is provided by comparing signals from a current sensor and a voltage sensor in a comparator. Depending upon the relative size of the compared signals, the switch is turned off or on.
Domshy, et al., U.S. Pat. No. 3,593,078, discloses a starting and operating control circuit that includes voltage sensors and current sensors to start a motor and limit the power supplied to it.
One design of a starter includes an integrated starter and protection circuit to detect fault conditions of an associated electrical device and, in response to detecting a fault condition, disable the electrical device. When a starter with an integrated starter and protection circuit is used to control devices in an environment in which the electrical devices are not dependent upon other machines, or their operation does not impact other devices, the inclusion of the integrated starter and protection circuit is an effective way to protect the associated electrical device. However, when a starter including such an integrated protection circuit is used in a system where the associated electrical device is interdependent with other electrical devices, use of such a starter to independently enable and disable the electrical device may result in devastating effects to the entire system. In other words, where several electrical devices combine to produce a result, as in systems that include a computerized control panel or system controller, it is desirable for the control panel or system controller to maintain control over the status of the electrical devices to minimize potential problems.
However, many starter designs do not include an integrated protection circuit and also include no-fault detection. To provide fault detection for electrical devices, a current sensor surrounding the power cable to the electrical device may be used to sense the electrical load current. The current sensor may be electrically interconnected between a control panel and the power cable to provide a signal to the control panel representative of the current within the power cable. Suitable current sensors are known, such as the current sensor sold by Veris Industries, Inc., under the trademark HAWKEYE 700. It is desirable to locate such a current sensor within a housing where a starter for the electrical device is located.
A relay is typically electrically interconnected between a remotely located control panel and a starter to receive a control signal from the control panel and in response to provide an indication to the starter. Like the current sensor, such a relay may be located within a starter housing.
The relay and current sensor in the past have been separate individual devices, each requiring a mounting location that may not be available within the starter housing due to limited space. Additionally, it takes additional time to install two separate devices and requires stocking two separate replacement parts for use in the event of device failure. Also, troubleshooting likely requires the inspection of both devices.
What is desired, therefore, is a protection device that is suitable for addition to existing electrical systems that operate in an environment including a control panel and a starter controlling the operation of an associated electrical device. The protection device should be compact, inexpensive, and fast to install.